This proposal examines the role of nitric oxide (NO) as a mediator of the cellular and molecular events that promote dilation in the cerebrovasculature. Nitric oxide synthases (NOS) are found within blood vessels and brain parenchyma, and both tissues are potential sources of NO. Linkages between NO, vasodilation and increases in relative cerebral blood flow (rCBF) during hypercapnia and brain metabolism have been established by experiments using L-arginine analogues that inhibit NOS. We propose to develop and characterize mutant mice that lack the neuronal NOS gene, KN (knock-out, neuronal) mice. Ongoing work in our laboratory is devoted to similar development of mice that lack the vascular NOS gene, KV (knock-out, vascular) mice. This molecular genetics approach allows us to distinguish between the relative contributions of the various NOS enzymes (neuronal vs. endothelial). We plan to use KN and KV mice to study the role of NO in normal rCBF regulation and during ischemia in intact animals. These are unique experiments that will supplement and extend existing pharmacological data intended to resolve existing controversies concerning whether NO mediates rCBF increases during physiological pertubations or pathophysiological events, whether neuronal or vascular pools mediate the responses, and whether neuronal and/or vascular NO determines tissue outcome in both focal and global ischemia.